Comment 1 for bug 911280

DB_HEAP is supported as a table type, and the 2 methods
    db->set_heapsize()
    db->set_heap_regionsize()
to set the parameters for a "heap" database can be configured.

That's enough for development needs to try-and-see whether
DB_HEAP is usefully supported. A "heap" has less overhead than
hash/btree for certain types of storage is the only perceived benefit,
and there are other non-trivial costs involved in changing to use DB_HEAP.